add margin auto

[GuidoOffermans]

Fixes #468

Summary

Added margin auto to center the block.

Relevant technical choices:

Test instructions

This PR can be tested by following these steps:

1. Create a new post.
2. add a block-quote.
3. Check if the alignment is correct in the editor.

Screenshots

If this is a visual change please include screenshots of the change at various screen sizes.

Desktop (Extra Large)

Desktop (Large)

Tablet (Medium)

Mobile (Small)

Quality assurance

• I have thought about any security implications this code might add.
• I have checked that this code doesn't impact performance (greatly).
• I have tested this code to the best of my abilities
• I have checked that this code does not affect the accessibility negatively

[aristath]

LGTM 👍
Confirmed fix after running npm run build